Carbon Dioxide (Urban CO2 Dome - Cities Outside U.S.) -- Summary
Following the discovery and characterization of the urban CO2 dome of Phoenix, Arizona, USA, studies of urban CO2 domes began to be conducted in many other parts of the world. In this summary, we describe the results of investigations of this phenomenon that have been conducted in cities outside the borders of the United States, proceeding in alphabetical order by country.

In Denmark, Soegaard and Moller-Jensen (2003) studied the urban CO2 dome of Copenhagen via (1) eddy covariance measurements made atop a 40-m mast located in the center of the city that logged data for more than 14,500 half-hourly periods that were evenly distributed over an entire year, (2) a second eddy covariance system located atop a 9-m telescopic mast mounted on a vehicle giving an effective height of 10 m that was operated periodically in different urban settings, including a major entrance road and residential and industrial areas, and (3) mobile transects made during the winter. As they describe it, this effort indicated that "traffic is the largest single CO2 source in the city," with the mobile measurements demonstrating that "emission rates range from less than 0.8 g CO2 m-2 h-1 in the residential areas up to a maximum of 16 g CO2 m-2 h-1 along the major entrance roads in the city center." In the summer, in fact, they report that "86% of the variance in the CO2-flux is explained by the traffic intensity." They also report that summertime traffic "accounts for 51% of the total CO2 emission, whereas in winter the contribution from traffic declines to 39% of the total emission due to the increase in local heating." In addition, their mobile transects in the winter revealed that city center CO2 concentrations were "50% higher than in the surroundings."

In France, Widory and Javoy (2003) measured CO2 concentrations and carbon isotope abundances in air samples collected from various locations (streets, gardens, etc.) in Paris, its suburbs and the surrounding open countryside, as well as from vehicles, heating sources, power stations, etc., and from laboratories and classrooms where the human component of the urban CO2 dome would be expected to be dominant. These data sets revealed that near-surface atmospheric CO2 concentrations throughout the country outside Paris averaged 415 ppm, while values in the city sometimes reached as high as 950 ppm. These higher values were driven primarily by vehicular exhaust, the two researchers noting that "road traffic is the main contributor and, in particular, vehicles using unleaded gasoline (~90% of the total)." In enclosed spaces, however, CO2 derived from human respiration was dominant. In a 150-m3 classroom that held 20 students for a period of four hours, for example, the indoor CO2 concentration rose as high as 4,630 ppm, which "corresponds well," in Widory and Javoy's words, "to an average individual respiration flux of 5 l/min containing ~3.7% CO2."

Also in France, Lichtfouse et al. (2003) collected samples of the aerial parts of several species of grass growing within 2 to 40 meters of a major highway in Paris that was traveled by about 8,000,000 vehicles per day, as well as samples of grasses that grew in remote rural areas of the country, from which δ13C values were obtained that allowed them to calculate the relative contributions of background atmospheric carbon and fossil-fuel-derived carbon to the plants' growth. From this information they determined that fossil-fuel-derived carbon comprised 23.3 ± 0.8% of the total carbon in the urban grasses, which suggests that during the daylight hours of this time of year (May), near-surface atmospheric CO2 concentrations at the studied site were likely elevated by about the same percentage above background rural values.

In comparison, over the complete daylight period of the same time of year in a suburban residential area of Phoenix, Arizona, USA, atmospheric CO2 concentrations averaged 10% more than the surrounding rural values (Idso et al., 2002). The higher values implied for Paris are likely due to sampling close to a heavily-traveled highway, as well as the fact that Paris' urban CO2 dome may be even stronger than that of Phoenix, which is suggested by the fact that maximum central-city CO2 concentrations in Phoenix were measured by Idso et al. (2002) to be on the order of 620 ppm, while those in Paris were measured by Widory and Javoy (2003) to at times have been as high as 950 ppm.

In Italy, Gratani and Varone (2005) measured atmospheric CO2 concentrations at various places throughout Rome in 1995, 1998 and 2001-2004. The data they obtained pointed to the existence of a strong but variable urban CO2 dome. Major characteristics of the dome included significant temporal influences (seasonal, weekly and diurnal), with higher CO2 concentrations being observed in the winter months (15% greater than summer months), on weekdays (22% greater than weekends), and in the morning hours (23% greater than afternoon hours). Spatially, CO2 concentrations at the periphery of the city averaged 405 ppm and 377 ppm on weekdays and weekends, respectively, whereas at the city center they averaged 505 ppm and 414 ppm on weekdays and weekends, respectively. In addition, the two scientists report a strong correlation between traffic density and CO2 concentration; and they note that CO2 concentrations were generally highest during times of maximum traffic density. Lastly, they report that over the 9-year period (1995-2004) there was a 30% increase in the city's near-surface atmospheric CO2 concentration, the reasons for which they do not directly address. However, they say that Rome has become increasingly urbanized over the past few years, and that many new suburbs have rapidly changed it into a mega-city of nearly three million inhabitants.

In Japan, Moriwaki et al. (2006) investigated temporal changes in vertical profiles of wind, temperature and atmospheric CO2 concentration on 60 winter days within a low-storied (mean house height of 7.3 m) residential sector of Kugahara, Tokyo, via measurements made at various levels along a tower erected in the backyard of one of the sector's homes, starting at 0.7 m above ground level (agl) and ending at 29 m agl. This work revealed that on a diurnal basis, the average CO2 concentration at 29 m agl ranged from 406 to 444 ppm, with the minimum value occurring at approximately 1400 hours and the maximum occurring around midnight, which values are to be compared to a background rural value of 380 ppm. During periods of calm and stably stratified conditions, however, midnight maximum concentrations sometimes exceeded 500 ppm. In addition, there was a small morning peak in CO2 concentration at about 0800 hours, which they attributed to "the increase of fossil fuel consumption in houses and traffic."

With respect to the vertical profile of CO2, the researchers report that above what they call the "suburban canopy," the air's CO2 concentration decreased rapidly with height. Within the canopy, however, they say CO2 concentration varied but little with height, "which indicates that the CO2 emitted from the houses accumulated within the canopy." More specifically, and noting that indoor CO2 concentrations sometimes exceeded 1000 ppm, they opined that a nighttime cold-air subsidence flow from the neighborhood's nocturnally-cooling rooftops brought down air parcels with high CO2 concentrations that originated at the outlets of building ventilating fans located in the middle to upper parts of the suburban canopy.

In Kuwait, Nasrallah et al. (2003) analyzed measurements of atmospheric CO2 concentrations and ancillary meteorological data that had been made at a stationary site in the Al-Jahra suburb of Kuwait City, Kuwait - at hourly intervals from 17 June 1996 to the time of the writing of their paper (31 May 2001) - with a view to comparing their results with those of similar analyses carried out over the prior few years in the Tempe suburb of Phoenix, Arizona, USA (Idso et al., 1998, 2001, 2002). Their findings indicated that (1) both locations exhibit an annual atmospheric CO2 concentration cycle consistent with patterns of plant growth and decay in the Northern Hemisphere and vehicular traffic fluctuations through the year, (2) both locations have a significant weekly CO2 cycle reflecting the decline in vehicular traffic during the designated weekend period, and (3) both cities exhibit a strong diurnal CO2 cycle created by patterns in vehicular traffic through the day and the diurnal cycles of local circulation and atmospheric stability. However, whereas the atmospheric CO2 concentration in Kuwait City ranged from a mean of 368 ppm in the afternoon to only slightly more than 371 ppm two hours prior to midnight, comparable values for Phoenix were 390 ppm and more than 450 ppm, respectively. One possible reason for the huge difference in absolute magnitude between the two locations may reside in the fact that measurements at Phoenix were carried out at a height of only one meter above the ground, whereas those at Kuwait City were made atop a 3-meter tower that stood on the roof of a 7-meter-tall building.

In Mexico, Velasco et al. (2005) installed an eddy covariance flux system at a height of 37 meters above the ground in a densely populated section of the city, where CO2 flux data were obtained over 23 days (7-29 April 2003) during the warm dry season, including a period of significantly reduced traffic during the Holy Week national holiday (14-20 April). The CO2 concentration data they collected showed a clear diurnal pattern over the period of study with the highest concentrations (398 to 444 ppm, average of 421 ppm) occurring during the morning hours and the lowest concentrations (average of 375 ppm) occurring in the afternoon. The average CO2 flux over the entire 23-day period was predominately positive, indicating that the surrounding urban surface was a net source of CO2 to the atmosphere, and that CO2 uptake by urban vegetation was not sufficient to offset the CO2 coming from anthropogenic sources. The five researchers also found that urban CO2 concentrations were an average of 20 ppm higher during the morning rush hour of the higher-traffic week prior to Holy Week and 14 ppm higher during the higher-traffic week following Holy Week. CO2 concentrations were additionally found to be higher on weekdays than on weekends, which effect, in the words of Velasco et al., is "directly related to vehicular traffic," since the transportation sector accounts for approximately 60% of the urban CO2 emissions burden.

Last of all, at four different times of the year in Poland, Zimnoch et al. (2004) made diurnal measurements of the concentration and isotopic composition (δ13C, δ18O) of atmospheric CO2 at a height of 20 meters above the ground at a location in the western part of Krakow that borders recreation and sports grounds and is some distance away from direct low-emission CO2 sources and strong car traffic. During the winter, as they describe it, they found that "the local CO2 contribution is made up almost exclusively from anthropogenic emissions," which produce nighttime atmospheric CO2 concentrations at 20 meters above ground level that are sometimes as high as 440 ppm, although they say "the calculated mean contribution of anthropogenic CO2 to the city atmosphere during winter is about 30 ppm." During the daytime in summer, however, they report that "the dominant source of CO2 is the local biosphere," and that "when the lower atmosphere was intensely mixed, the recorded CO2 concentration dropped to values close to those observed at 'clean' continental stations (354 ppm)." Nevertheless, on some summer evenings CO2 emissions from respiration and biomass decomposition would cause the air's CO2 concentration to rise as much as anthropogenic CO2 emissions would cause it to rise in the winter.

In considering the results obtained from the studies that have been conducted in these several cities from all across the world, as well as those obtained from cities within the United States, we see several commonalities. Anthropogenic CO2 emissions are the primary source of the urban CO2 dome, the dome is generally stronger in city centers, in winter, on weekdays, at night, under conditions of heavy traffic, close to the ground, with little to no wind, in the presence of strong temperature inversions. In addition, there are indications that the higher CO2 concentrations and temperatures found within the dome tend to enhance the productivity of urban vegetation.

References
Gratani, L. and Varone, L. 2005. Daily and seasonal variation of CO2 in the city of Rome in relationship with the traffic volume. Atmospheric Environment39: 2619-2624.